The wet etching process is a core art of fabricating gate, source-drain and pixel electrode of a semiconductor device by using an acid etchant to pattern metal layers. In general, aluminum and molybdenum are often used as the metal layers to form the gate pattern, a wet etching process of the aluminum and molybdenum layers can use a variety of acids; however, a mixed acid such as a composition of phosphoric acid, nitric acid and acetic acid is the most popular etchant for use in the wet etching process to dissolve and redox the aluminum and molybdenum layers, so that the gate pattern can be obtained.
A mixed acid of an aluminum etchant is consisted of phosphoric acid (about 70˜72%), nitric acid (about 1.8˜2.0%) and acetic acid (about 9.5˜10.5%), a reaction formula of each acid in the mixed acid is illustrated as follows:
                    ⁢        ⁢            +                        H          2                ⁢        O              →                  ⁢                  O          +                    +              NO        3        -                                2        ⁢                                  ⁢        Al            +              6        ⁢                  H          +                      →                  2        ⁢                                  ⁢        A        ⁢            +              3        ⁢                                2        ⁢                                  ⁢        Mo            +              6        ⁢                                  ⁢                  H          +                      →                  2        ⁢                                  ⁢        M        ⁢            +              3        ⁢                                ⁢        P        ⁢            +              2        ⁢                                  ⁢                  H          2                ⁢        O              →                  ⁢        ⁢                  O          +                    +              HP        ⁢                                2        ⁢                                  ⁢                              A            ⁢            l                                3            +                              +              ⁢        ⁢        3        ⁢                              H            ⁢            P            ⁢            O                    4                      2            -                          ⁢              →                  Al        2            (                        ⁢          ⁢          ⁢                →                                            2              ⁢                                                          ⁢              AlP              ⁢                        +                          ⁢              P              ⁢              ⁢                                                          ⁢              2              ⁢                                                          ⁢              M              ⁢                        +                          3              ⁢                                                          ⁢              HP              ⁢                                →                                    Mo              2                        (                                          HP                ⁢                ⁢                            →                                                2                  ⁢                                                                          ⁢                                      MoPO                    4                                                  +                                  ⁢                                      PO                    4                                                                                          
In the above reaction formulas, the nitric acid is as a role of providing H3O+ and etching alumina, the phosphoric acid provides phosphate to form a dissolvable complex with metal oxides, and the acetic acid can adhere on a surface of the reactant to reduce viscosity of the aluminum etchant so as to enhance wettability and adjust the etching rate. In the etching process, adjusting concentration of the nitric acid and the acetic acid is a key point of etching rate and pattern integrity.
Based on the acid-base theory, acidity of an acid or alkalinity of an alkali in solution is not only pertaining to the acid-base properties of themselves, but also related to a solvent of the solution. In aqueous solution, a difference of the ionization constant (pKa value) of the respective solutes therein is up to 5 so that the respective solutes can be measured by titration. For example, in a mixed acid solution consisted of nitric acid, phosphoric acid and acetic acid, the pKa of the nitric acid is −1.32, the first degree ionization constant (pKa1) of the phosphoric acid is 1.96, concentrations of the nitric acid and phosphoric acid cannot be respectively measured by titration due to the leveling effect formed between the nitric acid and the phosphoric acid; similarly, and the pKa of the acetic acid is 4.73, the second degree ionization constant (pKa2) of the phosphoric acid is 7.12, concentrations of the acetic acid and phosphoric acid cannot be respectively measured by titration. In an appropriated non-aqueous solution, a difference of the ionization constant (pKa value) of the respective solutes therein is up about to 2˜3 so that the respective solutes can be measured by titration. Therefore, an appropriated non-aqueous solvent is needed for use in titration of a mixed acid solution.
Due to high sensitivity, high accuracy, automation and continuous measurement, a potentiometric titration that determines a titration end point with a potential jump occurring in titration has been widely used. However, a conventional potentiometric titration of a mixed acid is mostly based on a two-steps method which is performed with a non-aqueous solvent. The conventional potentiometric titration includes: step 1, a content of a nitric acid in a mixed acid is titrated with tetrabutylammonium bromide as a titrant dissolved in ethanol solution and ethanol as a solvent, or KOH as a titrant dissolved in isopropanol solution and methanol as a solvent; step 2, contents of phosphoric acid and acetic acid are titrated with NaOH solution as a titrant and a saturated sodium chloride solution as a solvent. Two titration end points are sequentially occurred in the two-steps method, wherein the first titration end point is related to the nitric acid and the first hydrogen ion of the phosphoric acid, the second titration end point is related to the second hydrogen ion of the phosphoric acid and acetic acid, and then contents of the three acids in the mixed acid are obtained with automatic measurement. The conventional potentiometric titration which is performed with the two-steps method such as the above description has cumbersome and time-consuming process and a variety of titrants with solvents, so that the titration results are come with increased uncertain, reduced accuracy and worse reproducibility.